Techniques for optically compressing light intensity ranges

ABSTRACT

A pin hole camera assembly for use in viewing an object having a relatively large light intensity range, for example a crucible containing molten uranium in an atomic vapor laser isotope separator (AVLIS) system is disclosed herein. The assembly includes means for optically compressing the light intensity range appearing at its input sufficient to make it receivable and decipherable by a standard video camera. A number of different means for compressing the intensity range are disclosed. These include the use of photogray glass, the use of a pair of interference filters, and the utilization of a new liquid crystal notch filter in combination with an interference filter.

FIELD OF THE INVENTION

The United States Government has rights in this invention pursuant tocontract number W-7405-ENG-48 between the United States Department ofEnergy and the University of California for the operation of theLawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

The present invention relates generally to techniques for compressingrelatively large light intensity ranges and more particularly todifferent optical techniques for accomplishing this utilizing (1)photogray glass, (2) interference filters, and (3) a novel liquidcrystal notch filter, the latter by itself being the subject of thepresent invention.

There are times when it is desirable to view an object having arelatively large light intensity range and record what is viewed bymeans of a camera or like instrument. For example, applicant has found aneed to view the crucible containing molten uranium in an atomic vaporlaser isotope separation (AVLIS). However, this molten uraniumcontaining crucible functions as a black body having a center which isat a temperature of approximately 3800° k and outer edges attemperatures on the order of 1000° k. Thus, the temperature range acrossthis black body is approximately 3000° k. and has a correspondinglylarge light intensity range (9 orders of magnitude) which is much toolarge for a standard video (2-3 orders of magnitude) camera or likeinstrument to receive and decipher. Therefore, if standard viewingand/or recording equipment is to be used to view the crucible, its lightintensity range must be compressed (1-3 orders of magnitude) which is atolerable level.

The concept of compressing a relatively large light intensity range isnot new. Heretofore it has been done electronically, that is, byconverting the incoming light to corresponding electrical signals,compressing the electrical signals, and then converting those compressedelectrical signals back to light which itself is compressed relative tothe incoming light. Applicant has found this "electronic" approach tolight compression to be relatively complicated and expensive.

To applicant's knowledge there has been no suggestion of compressing alight intensity range entirely optically, that is, by acting on thelight itself rather than converting the light to electrical signalswhich are then compressed and converted back to light.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide an entirely optical technique for compressing a light intensityrange.

Another object of the present invention is to optically compress lightintensity ranges in a number of different uncomplicated and economicalways.

A more particular object of the present invention is to provide a cameraassembly including standard camera recording equipment for viewing andrecording an object having a light intensity range which is normallyoutside the intensity range capabilities of the standard recordingequipment used by the camera.

Another specific object of the present invention is to provide the lastmentioned camera assembly with an entirely optical mean for compressinglight intensity ranges which are otherwise too large for the cameraassembly's recording equipment.

A further object of the present invention is to provide a novel liquidcrystal notch filter which is especially suitable for use as a componentin an entirely optical technique for compressing light intensity ranges.

As indicated immediately above, one aspect of the present invention isdirected to a technique for compressing a light intensity range entirelyoptically. This is accomplished by utilizing a light filteringarrangement. In accordance with one embodiment of the present invention,the light filtering means includes a photogray lens. In accordance withanother embodiment, the light filtering means includes a pair ofserially spaced interference filters and in accordance with stillanother embodiment, the light filtering means includes a singleinterference filter in combination with a liquid crystal notch filterwhich itself is designed in accordance with the present invention.

The liquid crystal notch filter just mentioned i of a general type knownin the art, that is, a filter configured to pass light at allwavelengths except for light at a relatively narrow wavelength bandwhich defines the filter's notch. In accordance with the presentinvention, this otherwise readily available filter is modified in a waywhich causes the wavelength band to vary, at least to a limited extent,with temperature. This temperature sensitive liquid crystal notch filterand a known interference filter together form a light filteringarrangement which is especially suitable for compressing light intensityranges of the type associated with molten uranium containing crucibles,that is, light intensity ranges associated with temperature ranges onthe order of 2000° k-3000° k.

Additional objects, advantages and novel features of the invention willbe set forth in the description which follows and in part will becomeapparent to those skilled in the art upon examination of the followingor may be learned by practice of the invention. The objects andadvantage of the invention may be realized and attained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the present invention recited briefly above willbe described in more detail hereinafter in conjunction with the drawingswherein:

FIG. 1 is a diagrammatic illustration of a pin hole camera including anassembly designed in accordance with the present invention for opticallycompressing the light intensity range from an object being viewed by thecamera;

FIG. 2 diagrammatically illustrates a photogray lens which serves as theoptical light compression assembly forming part of the cameraillustrated in FIG. 1;

FIG. 3 graphically illustrates how the photogray lens of FIG. 2functions to compress the light intensity range appearing at its frontface;

FIG. 4 diagrammatically illustrates the utilization of two interferencefilters which cooperate with one another to form a second type ofoptical light compression assembly forming part of the cameraillustrated in FIG. 1;

FIGS. 5A, 5B and 5C and FIGS. 6A, 6B and 6C illustrate how theinterference filters of FIG. 4 function to compress a light intensityrange appearing at the front face of one of the interference filters;

FIG. 7 diagrammatically illustrates an interference filter incombination with a liquid crystal notch filter designed in accordancewith the present invention such that the two together form a third typeof optical assembly for compressing a light intensity range appearing atthe front face of the liquid crystal notch filter;

FIG. 8 diagrammatically illustrates how the liquid crystal notch filterfunctions apart from the interference filter in accordance with thepresent invention; and

FIGS. 9A, 9B, 9C, 10A, 10B and 10C diagrammatically illustrate how theliquid crystal notch filter and interference filter function together tocompress a light intensity range appearing at the front face of theliquid crystal notch filter.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to preferred embodiments ofthe-invention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in detail in connectionwith these preferred embodiments, it will be understood that it is notintended to limit the invention to those embodiments. On the contrary,it is intended to cover all alternative modifications and equivalents asmay be included within the spirit and scope of the invention as definedby the appended claims.

Turning now specifically to the drawings, attention is first directed toFIG. 1 which illustrates a pin hole camera assembly which is generallyindicated by the reference numeral 10 and which includes two sections, acamera section 12 and a separate and distinct optical section 14. Thecamera section, which is only partially shown, may be of any suitableknown kind, in its entirety, or it may be identical to the correspondingcamera section in applicant's pending U.S. patent application Ser. No.863,911, filed simultaneously with the present application and entitledpin hole camera assembly having X-ray blocking capabilities. In eithercase, the camera section includes a camera 15 and a lens assembly or eyepiece 16 which may have a fixed focus or one which is variable as in thecopending patent application just recited. Optics section 14 includes asubsection 18 fixedly mounted in spaced-apart coaxial relationship withthe input to camera section 12 by suitable means such as coupling 20 anda subsection 22 which extends perpendicular to subsection 18 and whichdefines a pin hole aperture 24. The optics section 14 may include anysuitable optics to reproduce an image of its aperture 24 at theentrance. Pupil cf camera section 12. In the particular embodimentillustrated, this is accomplished in the same way as the correspondingoptics section in the above recited copending patent application.Specifically, an inclined mirror or other suitable light reflectingsurface 26 is fixedly disposed at the juncture between subsections 18and 22 for directing light entering aperture 24 through subsection 18 inthe direction of camera section 12. Spaced-apart eye pieces 28 and 30located within subsection 18 relay this light to an image of theaperture generally indicated at 32 on the entrance pupil of camerasection 12. Also eye piece 28 forms an image of the source 12 on anintensity compressor arrangement to be discussed hereinafter.

Inasmuch as overall camera assembly 10 illustrated in FIG. 1 isespecially suitable for viewing an object which includes a source ofx-radiation, specifically molten and evaporating uranium in a crucibleforming part of an AVLIS system, the light reflecting surface 26 ispreferably designed to absorb the x-radiation and follows a gas purgedhole 32 where the purge gas repels uranium evaporant hence maintaining ahighly reflective surface 26, as described in the above recitedcopending application. In FIG. 1, the visible light entering aperture 24of optics section 14 is diagrammatically illustrated by dotted lines at34 while x-radiation entering the aperture is diagrammaticallyillustrated by wavy line 36. Both are shown emanating from an object 38being viewed by the camera assembly.

Camera assembly 10 is designed especially for viewing an object having arelatively large light intensity range, specifically a range which istypically outside the normal capabilities (2-3 orders of magnitude) ofstandard camera equipment to decipher and record. One such object ismolten uranium in a crucible where the temperature rang across thecrucible is as high as 3000° k, thereby resulting in a correspondinglylarge light intensity range of 6-9 orders of magnitude. To this end,camera assembly 10 includes an arrangement for compressing (by 1-3orders of magnitude) the light intensity range being viewed before itreaches camera section 12. This arrangement which is generally indicatedat IC 40 is shown forming part of overall optics section 14. As will beseen hereinafter, this arrangement can take the form of severaldifferent embodiments, although in each case, it functions to compressthe light intensity range being viewed by the camera assembly entirelyby means of optics utilizing a light filtering technique. This is incontrast to known electronic compression techniques such as the onedescribed previously.

Turning now to FIG. 2, one specific embodiment of compressionarrangement IC 40 is illustrated there at 40'. This latter arrangementwhich is known in the art includes a photogray lens 42 having a frontface 44 and a back face 46. Photogray glass becomes dark within aspecific wavelength range and a transparent filter after the photograyglass maximizes observing the range of transmission that the photograyglass can achieve. Infrared cut filters can be placed at the input ofthe photogray in order to reduce erasure of the UV light written image,which could reduce overall contrast achievable and image life time.

For purposes of discussion, a portion of photogray lens 42 will bedivided into four segments numbers 1-4. FIG. 3 graphically illustrates alight intensity pattern which might appear at the front face 44 of thephotogray lens. This pattern which is generally indicated at thereference numeral 50 represents visible light. As seen in FIG. 3, themaximum intensity of pattern 50 appears across section 2 of thephotogray lens with lesser intensities appearing across sections 1, 3and 4.

It is the nature of photogray glass to "darken" to a greater extent inthose areas receiving more UV light than in those areas receiving lesserUV light. Therefore, in the example illustrated by pattern 50 in FIG. 3,section 2 of the photogray lens will darken to a greater extent thansections 1, 3 and 4, thereby filtering out more of the light trying topass through section 2 than the other sections. The resultant lightpattern appearing at the back face of the photogray lens has acompressed intensity range as shown at 52. Note that the intensitydifferences across section 2 of the lens between curve 50 and 52 aregreater than the intensity differences between the two curves along theother sections. Thus, while the intensity range represented by lightpattern 50 may be too large for standard camera recording and/orreviewing equipment, the intensity range corresponding to pattern 52 maybe acceptable. In this latter regard, it should be noted that thecompressed pattern 52 not only has a different (smaller) intensity rangethan uncompressed pattern 50 but its intensity distribution across thelens has been changed resulting in grey scale distortion of the scenebeing viewed and therefore the use of arrangement 40' (or any of theother arrangements to be described) would not be suitable where it isnecessary to preserve the grey scale being viewed. However, for thepurpose of viewing a crucible containing molten uranium in an AVLISsystem in order to monitor molten areas, arrangement 40' (and any otherarrangements to be described) can be used since a change in intensitydistribution is not critical. Further, in the case where arrangement 40'is used to monitor a crucible containing molten uranium it is onlynecessary to monitor a particular wavelength of light, for example thewavelength λ0.

While the use of a photogray lens is certainly substantially lesscomplicated than the electronic approach described previously forcompressing a light intensity range, it does have a disadvantage whichmay not make it entirely practical for viewing a crucible containingmolten uranium. Specifically, photogray glass requires ultra-violetradiation to activate its filtering characteristics. There is not alarge amount of ultra-violet light in the intensity distribution in acrucible containing molten uranium. Therefore, while such an arrangementmay be a suitable technique in pin hole cameras designed to look atobjects which have ultra-violet radiation, for example the sun, they arenot an entirely satisfactory approach for use in viewing ultra-violetfree objects or low ultra-violet objects such as uranium containingcrucibles.

Turning now to FIG. 4, attention is directed to an arrangement forcompressing a light intensity range designed in accordance with a secondembodiment of the present invention and one which does not have thedisadvantage associated with arrangement 40', as discussed immediatelyabove. This second arrangement which is generally indicated at 40" inFIG. 4 includes a pair of spaced-apart interference filters IF₁ and IF₂which are in optical series with one another. The interference filterIF₁ is positioned immediately in front of interference filter IF₂ withrespect to the incoming light generally indicated at 54. For purposes ofdiscussion, interference filter IF₁ is shown including a front face 56and a back face 58 and the interference filter IF₂ is shown including afront face 60 and a back face 62 for purposes of description, IF,interference filters is separated into four sections (area wise),sections 1, 2, 3 and 4, as was arrangement 40'.

An interference filter is a known and readily available optical devicewhich does not transmit any appreciable light, except within a verynarrow pass band which shifts to a limited extent in response to changesin temperature. The interference filter may be designed to pass aspecific wavelength band and shift in a particular way with temperatureor, at least, the temperature responsive shift can be readilycalibrated. FIG. 5A graphically illustrates the transmissioncharacteristics of an interference filter IF₁ while FIG. 5B graphicallyillustrates the transmission characteristics for an interference filterIF₂ forming overall arrangement 40". At room temperature theinterference filter IF₁ passes only light within a very narrow λ1 bandwhile interference filter IF₂ passes only light in a very narrow λ2band. Note, that the two bands λ1 and λ2 overlap when the twointerference filters are positioned in serial relationship to oneanother optically, as illustrated in FIG. 4, such that the resultantpass band is even a narrower λ0 band which is shown in FIG. 5C. Thus, atroom temperature (for example 68° F.), each section 1, 2, 3 and 4 ofarrangement 40" passes light in the narrow wavelength band of λ0 only.

Referring specifically to FIG. 6, the two transmission bands λ1 and λ2corresponding to interference filters IF₁ and IF₂, respectively, areshown in overlapped fashion so as to combine to define the ultimate passband λ0 which is shown shaded in FIG. 6. This shaded λ0 band resultswhen the interference filters are at room temperature. If the pass bandof one of these filters is subjected to changes in temperature so as tocause its pass band to shift while the other remains at roomtemperature, the resultant λ0 band will either decrease in size orincrease in size. For example, let it be assumed that the interferencefilter IF_(I) is subjected to such temperature changes while theinterference filter IF₂ is not. If the temperature change is in onedirection, the pass band λ1 will shift in one direction, for example tothe left as illustrated in FIG. 6B, and if the temperature change is inthe opposite direction the pass band λ1 will shift in the oppositedirection, for example to the right as illustrated in FIG. 6C. When thepass band λ1 shifts in one direction, λ0 decreases in size causing lesslight to pass therethrough (and thereby compressing light intensity) andwhen λ1 shifts in the opposite direction, λ0 increases in size causingmore light to pass therethrough.

In the case of arrangement 40", the entire arrangement can be positionedwithin or as part of optics section 14 of camera assembly 10 but, in anycase, must be positioned so that the temperature across interferencefilter IF₂ remains constant, for example at room temperature, while thetemperature across interference filter IF₁ corresponds to or at least isdirectly proportional to the temperature range across the object beingviewed, for example a crucible containing molten uranium. Under thesecircumstances, the pass band λ1 associated with each section 1, 2 and soon of interference filter IF₁ will vary depending upon the temperatureat that section such that when combined with the corresponding sectionof interference filter IF₂, the amount of light passing through bothfilters will vary with the temperature at its front face in the mannerdiscussed above. As a result, assuming that the light intensity range atthe input to arrangement 40" corresponds to the temperature range at theobject being viewed, arrangement 40" will compress the light intensityrange.

It should be apparent from the foregoing that arrangement 40" is notdependent upon ultra-violet light as is arrangement 40'. However,arrangement 40" is dependent upon temperature and assumes that theobject being viewed has a light intensity range which corresponds to itstemperature range. It is also assumed that arrangement 40" can beappropriately located relative to the object being viewed such thatinterference filter IF₁ is subjected to a proportional temperature rangewhile interference filter IF₂ is maintained at a fixed temperature. Inthe case of a pin hole camera, applicant has found it difficult todevelop the temperature changes across the interference filter IF₁ inproportion to the actual temperature range associated with a cruciblecontaining molten uranium and therefore arrangement 40" is not the mostideal arrangement for use in viewing such an object. Since light thatcan heat filter IFI only transmits to the spacer layer in the band λ0(visible) some light in the infrared could reach the spacer layer.However the optics do not form sharp images for IR radiation.

A more ideal arrangement is illustrated in FIG. 7 and generallydesignated by the reference numeral 40'". This arrangement, likearrangement 40" includes an interference filter IF₂ as a second ordownstream filter forming part of a pair of filters. However, theupstream filter is not another interference filter but rather a liquidcrystal notch filter designed in accordance with the present invention.This latter filter, which will be described in more detail below andwhich is generally designated LCNF has a front face 70 and a back face72. As illustrated in FIG. 7, the liquid crystal notch filter ispositioned in front of and spaced from interference filter IF₂ so as tofirst receive input light from the object being viewed, as indicated bymeans of arrow 74. As will be described hereinafter, the notch filter isconfigured to vary with temperature and combines with the interferencefilter IF₂ which is intended to remain at a fixed temperature tocompress the light intensity range appearing at the front face of theliquid crystal notch filter. In this regard, for purposes ofdescription, the liquid crystal notch filter has been divided intoaligned sections 1, 2 3 and 4.

Before discussing how the overall compression arrangement 40'" functionsto compress the light intensity range of a light pattern appearing onthe front face of the liquid crystal notch filter, attention is directedto this latter filter per se. Liquid crystal notch filters generally areknown in the art. A detailed discussion of such a device appears in apublication entitled LIQUID CRYSTALS AS LARGE APERTURE WAVEPLATES ANDCIRCUITRY POLARIZERS by Stephen D. Jacobs dated 1981, SPIE Vol. 307, andtwo additional publications. They are CHOLESTERIC FILMS AS OPTICALFILTERS, James Adams, Werner Hass, and John Daily, Journal of AppliedPhysics Vol. 42, No. 10, September 1971, and LIQUID CRYSTAL LASERBLOCKING FILTERS, S. D. Jacobs and K. A. Cerqua, Laboratory for LaserEnergetics, Rochester N.Y., LLE Review, Vol. 15. Apr.-June 1983, Labreport No. 157. The liquid crystal notch filter (LCNF) described thereis one which passes all light, except for light in a very narrow band.For purposes herein, that band will be referred to as the filter's notchand may be designed to fall within a particular wavelength band. Thistype of device has been commercialized and advertised as a laserblocking filter, for example for use in laser goggles which are used toblock the wavelength of laser light by individuals who work with suchlight. It is important that these commercial devices be manufactured tobe thermally stable. For the protection of the user, the filter's notchcannot shift in wavelength for that would leave the user unprotectedwith respect to the laser light being handled. It is apparentlyrelatively easy to make laser goggles thermally stable since most of thelight passes through the filter, except for the light in the wavelengthdefining the notch. Thus, little if any infrared radiation will beabsorbed and the filter will not heat up. So long as the LCNF does notheat up, its notch will not shift.

While the prior art liquid crystal notch filter just described isparticularly suitable for use in laser goggles, its thermal stability isnot a desirable quality for use as part of overall compressionarrangement 40'". Quite to the contrary, the liquid crystal notch filterforming part of arrangement 40'" must be thermally responsive in acontrolled way, that is, in a way which causes its notch to shift eitherlinearly or in a way that can be readily calibrated (e.g., predicted).In accordance with one aspect of the present invention, the liquidcrystal notch filter is combined with suitable means that will make itabsorb infrared radiation and heat up, thereby causing its notch toshift red in response to this heat in a controlled manner. This is bestillustrated in FIG. 8 which shows a transmission curve as a function ofwavelength for such a liquid crystal notch filter, when the latter is atroom temperature. Note that at room temperature the filter transmitslight at all visible wavelengths, except for a relatively narrow band ofcircularly polarized light centering at λ0. At the same time, the liquidcrystal notch filter is designed to absorb radiation outside a 2 or 3FWHM wide region about the notch. The location of the absorption banddepends upon the anticipated infrared radiation band (e.g., temperaturerange) to which the liquid crystal notch filter is to be subjected to.For example, in the case of a metal containing crucible which displaystemperature extremes between 2000° K. and 4000° K., the absorption bandmight be short of 0.6 UN to 10 UM, while the notch (λ1) might be 0.5 UM.

One way to make the liquid crystal notch filter responsive totemperature is by doping it with a compatible component, for exampleknown organic dyes, so that the doped LCNF has an infrared absorptionband outside the notch region. However, the organic dye or other suchmeans must not-interfere with the transmission of light as does thenotch itself. Its role is simply to absorb light and produce heat dueto, for example, the organic molecules in the dye when dye is used asthe doping agent. A proposed working embodiment of a thermallyresponsive liquid crystal notch filter includes R B or crystal violetperclorhte as the doping agent. That particular LCNF has a notch at thewavelength 633 mm and the absorption band is outside the notchwavelength at 633 mm. This thermally responsive liquid crystal notchfilter is made in the following manner.

Mix a cholesteric liquid crystal with a dye that does not absorb muchlight at the central wavelength of the interference bandpass of choice.The mixture could be TM74A, B and TM75A, B from EM Industries Company,Hawthorn, N.Y., or similar cholesteric material (that will scatter one

handedness circularly polarized light) with a doping of 10⁻² -10⁻³ molardye of Rhodamine B or similar absorption spectrum dye if considering a480 nm badpass filter.

Place this CLC dye mixture between two thin pellicle (of for instance2-8 micron thick nitrocellulose

or mylar each) held apparent 6-36 micron by mylar spacers. This devicewill scatter light at the notch wavelength and absorb light at +-2 notchbandwidth about the notch wavelength.

The ambient temperature and CLC mixture are used to set the notchwavelength.

The input image locally heats the CLC dye device over a wavelength rangewhere the dye absorbs light. The dye is necessary since liquid crystaland pellicle have little absorption in the visible spectrum.

In order to form an intensity compressor the notch wavelength must be ata longer wavelength than the NBIF since the notch will usually move toshorter wavelength as the CLC dye mixture heats up.

The CLC must be aligned in a Grandjean structure. This can be done withalignment layers on the pellicle. They are produced by vacuumevaporating at large angles of incidence with SiO₂, but these structuresmay contain aligned surface molecules that will serve this function, orafter the CLC dye mixture is placed into the pellicle sandwich thepellicles can be sheared with respect to one another to force the CLC toalign in the Grandjean structure.

The notch filter function shown in FIG. 8 is unlike those previouslyadvertised, since light is absorbed in a linear polarizer. This is incontrast to two CLC in series that do not absorb light to form a notch.

Using two CLC in series forms the notch without absorbing any light. Twodifferent notch filter are possible using serial CLC; one uses two CLCof the same handedness CLC with a half wave plate between them as inFIG. 3 of the Adams et al publication recited previously; the secondusing opposite pitch CLC as in FIG. 1. Each has the disadvantage for thepresent purposes that two layers need be heated to get a notchwavelength shift. With two different pitch CLC used serially it would bedifficult for each CLC to have the same temperature dependent shifts totheir scattering properties allowing a notch: filter to change itswavelength with temperature change. With the same CLC pitch andintervening half waveplate the device mass makes it difficult to heatboth CLC layers following the intensity contours of the image.

Therefore one ca consider not tuning the notch wavelength (for all inputpolarizations) but instead creating or destroying it based on thewavelength alignment between the scattering wavelength of each serialCLC. This would simply require the heating of one of the CLC and not theother by doping the first CLC with dye and not the second CLC. This willthen allow the first CLC to heat in response to the input image on itssurface as was done above. In this case the two CLC are offset withrespect to one another at room temperature. The second CLC is at theNBIF wavelength and the first mixture, containing a dye doping, isadjusted to be at a longer wavelength than the NBIF. Now at roomtemperature the serial use of the two CLC and NBIF transmit onehandedness polarized light at the NBIF wavelength. As the first dyedoped CLC layer is heated by an input image its scattering wavelengthbegins to coincide with the second CLC. At perfect alignment no lighttransmits through the second CLC at the NBI wavelength.

FIG. 8 layout of a thermally activated intensity compressor. Light isincident from the left at al possible polarizations. A CLC doped withdye is held between pellicle of low mass to facilitate heating form aninput image. The CLC scatters one handedness polarized light over a20-40 nm bandwidth and transmits the other handedness polarization.Outside this bandwidth the CLC is transparent except for dye held in itsmixture heats the CLC due to visible light absorption in the dye.Following this device the circular light is made linear and absorbed bya linear polarizer. The pellicles holding the CLC the quarter waveplateand linear polarizer form a notch filter. The tuning of the notch filteris viewed by a narrow band interference filter NBIF or IF². As the notchfilter tunes past the NBIF the serial combination will not pass lightwhen aligned in wavelength. Since an image drives the CLC heating thedevice will be less transparent for the brighter part of the imagecompared to a dimmer portion of the image. The device functions tocompress an interscene intensity distribution and intensity range to thesame interscene distribution but of compressed intensity range. i.e.,the grey scale is modified.

As indicated above, the thermally responsive liquid crystal notch filterjust described has been designed in accordance with the presentinvention apart from its role in overall arrangement 40'". For example,such a filter could be used as a temperature sensing device. However, in

overall arrangement 40"' it combines with interference filter IF₂ inorder to compress the intensity range of light impinging on its frontface. The way this is accomplished is best illustrated in FIGS. 9A, 9Band 9C and 10A, 10B and 10C in conjunction with FIG. 8. FIG. 9A is agraphic illustration of the light transmissive characteristics of theLCNF as a function of wavelength at ambient temperature while FIG. 9B isa similar graphic illustration of the interference filter IF₂. Note thatthe notch at λ1 associated with the liquid crystal notch filter and thetransmission band λ2 of the interference filter, at room temperatureoverlap so as to provide a resultant pass band at λ3. FIG. 10A betterillustrates this by showing a notch λ1 and the pass band λ2 inoverlapping relationship so as to produce a resultant ambient pass bandλ3. This pass band appears at each of the sections 1 through 4 acrossthe two filters assuming that the temperature at each section of eachfilter is at the ambient temperature. If the temperature pattern acrossthe liquid crystal notch filter varies in a way corresponding to, forexample, the temperature across a molten uranium containing crucible,the λ1 notch will shift a corresponding amount. Thus, the notch mayshift to the right, for example in one section, as diagrammaticallyillustrated in FIG. 10B, and it may shift to the left in anothersection, as diagrammatically illustrated in FIG. 10C. In both of thesecases, it is important to note that the pass band λ2 associated witheach section of the interference filter IF₂ remains constant. This isbecause the temperature across the interference filter remains constant,i.e., at the ambient temperature. With this in mind, it can be seen thata right hand shift of notch λ1 causes the resultant pass band λ3 todecrease in size and thereby transmit less light while a shift to theleft increases λ3 and thereby increases the amount of light passingthrough that section. A shift to the right corresponds to a hottersection and a shift to the left corresponds to a cooler one, relativelyspeaking. Thus, the overall pattern of light passing through the twofilters has its light intensity range compressed in a way whichcorresponds to the temperature pattern across the front face of theliquid crystal notch filter. This technique is more ideal for use in apin hole type camera assembly of the type illustrated in FIG. 1 than isthe arrangement 40" because thermal radiation readily transmitted to theliquid crystal allowing the dye to heat the liquid crystal over a largewavelength range.

The foregoing description of the preferred embodiment of the inventionhas been presented for purposed of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed, and obviously many modifications and variations arepossible in light of the above teaching. The embodiment was shown anddescribed in order to best explain the principles of the invention andits practical application to thereby enable others skilled in the art tobest utilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended thereto.

What is claimed is:
 1. An optical filter device comprising a temperaturesensitive liquid crystal notch filter which transmits substantially alllight, but absorbs light at a wavelength band which defines the filter'slight absorption notch, chracterized by the fact that said filter'slight absorption notch shifts predictably in response to changes intemperature, said temperature sensitive liquid crystal notch filterhaving a narrow wavelength light absorption notch and additionallyhaving a predetermined infrared absorption band different from saidlight absorption notch, whereby the temperature of said liquid crystalnotch filter is caused to increase in the presence of light in saidinfrared absorption band and said light absorption notch is caused toshift predictably in response thereto.
 2. In a liquid crystal notchfilter of the type which passes light at all wavelengths except at arelatively small, fixed wavelength band which defines the filter'snotch, the improvement comprising means for modifying said liquidcrystal notch filter in a way which causes said wavelength band to shiftwith changes in temperature and wherein said means for modifiying saidliquid crystal notch filter includes means for providing a predeterminedinfrared absorption band different from said wavelength band definingsaid filter's notch whereby said liquid crystal notch filteradditionally absorbs infrared radiation in said predetermined infraredabsorption band and heated thereby.
 3. The improvement according toclaim 2 wherein said means for providing a predetermined infraredabsorption band different from said wavelength band defining saidfilter's notch includes a compatible doping substance incorporated intosaid liquid crystal notch filter.
 4. An optical filter device accordingto claim 1 wherein said temperature sensitive liquid crystal notchfilter is doped with an organic dye to provide said predeterminedinfrarred absorption band.
 5. The improvement according to claim 3wherein said compatible doping substance comprises an organic dye.